The distributed form of a project (especially one based on GNU autotools) is often not merely a tarball of the upstream repository. Often the author has pre-run some stages of the build that she feels would impose unnecessary or esoteric dependencies. Typically this involves pre-generating Makefile.in from Makefile.am and configure from configure.ac. Another common one is for the maintainer to pre-generate the documentation. The user of the distributed tarball is fully capable of re-running these steps, but they have become optional.
This is a fact often forgotten when projects switch to github and just start using the automated release tarballs github will make from the repo.
This is a fact often forgotten when projects switch to github and just start using the automated release tarballs github will make from the repo.